Method for producing a magnet with radial magnetic anisotropy

ABSTRACT

In production of a permanent magnet by forming a roll of an elongated magnetic metal strap, a strap made from a spinodal decomposition type magnetic alloy is subjected, at least before formation into a roll, to age-hardening under concurrent magnetization in order to obtain a magnet having significant radial magnetic anisotropy well suited for use in sound systems such as loudspeakers.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a magnet withradial magnetic anisotropy, and more particularly relates to producingof a novel magnet having radial magnetic anisotropy and well suited foruse in magnetic circuits in sound systems such as loudspeakers.

Production of a radially magnetized permanent magnet is disclosed in,for example, Japanese Patent Publication Sho. 45-8584, in which themagnet has different poles on the inner and outer radial end sections.In the production process of this prior proposal, a magnetic strap isformed by solidifying magnetic powders of ferrite or the like withrubber, the solidified magnetic strap is next magnetized in itsthickness direction, and the magnetized strap is wound up onto a tightroll having the required shape of a magnet.

In the case of this prior proposal, however, choice of the material usedfor forming the initial magnetic strap is limited to an easilydeformable magnetic material such as ferrite or the like solidified withrubber. These magnetic materials are in general isotropic in theirmagnetic properties and rather unsuited for production of magnets usedfor the above-described purposes.

SUMMARY OF THE INVENTION

It is the object of the present invention to enable easy production of anovel magnet having radial magnetic anisotropy and well suited for usein sound systems with freedom in choice of the material.

In accordance with the basic concept of the present invention, a thinstrap is formed from a magnetic alloy which can be provided withmagnetic anisotropy through spinodal decomposition etc, the thin strapis next subjected to age-hardening under magnetization in its thicknessdirection for provision of the magnetic anisotropy, and the strap afterthe age-hardening is taken up onto a tight roll of a prescribed shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified side view, partly in section, of one embodimentof the age-hardening process employed in production method of thepresent invention,

FIGS. 2 and 3 are plan views of the rolled strap obtained in theproduction method of the present invention,

FIGS. 4 to 6 are plan views of several examples of the magnet producedby the method of the present invention, and

FIG. 7 is a simplified side sectional view of a loudspeaker magneticcircuit incorporating a magnet produced by the method of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a thin strap is first formedfrom a magnetic alloy which can be provided with magnetic anisotropythrough spinodal decomposition etc. Fe--Cr--Co type magnetic alloys orCu--Ni--Fe type magnetic alloys (cunife, etc.) are used as typicalspinodal decomposition type magnetic alloys. The Fe--Cr--Co typemagnetic alloy may contain 2 to 30% by weight of Cr, 5 to 37% by weightof Co, and Fe substantially the balance. Conditionally, 0.1 to 8% byweight in full of one or more of Ti, Zr, Ni, V and Si may be containedalso. The Cu--Ni--Fe magnetic alloy may contain 10 to 30% by weight ofNi, 10 to 30% by weight of Fe, and Cu substantially the balance. Morepreferably, the alloy should contain 15 to 25% by weight of Ni, 15 to25% by weight of Fe, and Cu the balance.

Formation of the thin strap can be carried out in various ways alreadywell known to one skilled in the art. In one example of the process, aningot of the above-described alloy composition is prepared by casting.The ingot so prepared is deformed into a thin strap of a prescribedthickness by sequential application of hot forgoing, hot rolling andcold rolling. The thin strap so obtained is further subjected toso-called solution treatment which includes, in sequence, cold rolling,annealing and abrupt cooling. In the case of a Fe--Cr--Co type magneticalloy, for example, the strap is preferably annealed by heating at atemperature above 1050° C. for 5 to 8 minutes, and rapidly cooledthereafter.

The thin strap so prepared is next subjected to age-hardening undermagnetization in its thickness direction. One example of the process isshown in FIG. 1. An elongated, thin strap 1 of a magnetic alloy isdelivered continuously from a supply roll 2 and advanced through aheater tube 3. Different poles 4 of a magnetizer are arranged on bothradial sides of the heater tube 3 in order to magnetize the thin strap 1in its thickness direction during its travel through the heater tube 3.A cooling nozzle 5 is arranged near the outlet of the heater tube 3 forejection of air of room temperature. After delivery from the heater tube3, a magnetized strap 1' is taken up tightly onto a take-up roll 6.

This age-hardening provides the thin strap with magnetic anisotropy inits thickness direction. That is, in the case of spinodal decompositiontype magnetic alloys, highly magnetic phase fractions and nonmagneticphase fractions are separated through spinodal decomposition caused bymagnetization under heat application and the highly magnetic fractionsphase are elongated in the thickness direction of the strap, therebyproviding excellent magnetic anisotropy in the thickness direction.

The age-hardening under magnetization should be carried out at a heatingcondition conducive to spinodal decomposition of the magnetic alloys.More specifically, the initial heating is advantageously carried out ata temperature in a range from 670° to 720° C. for a period in a rangefrom 10 to 60 minutes, and subsequent cooling down to 600° to 620° C. iscarried out at a rate of 10° to 90° C./hr. After the subsequent coolingis complete, the magnetized strap is cooled rapidly.

The intensity of the magnetic field at the magnetization forage-hardening should preferably be in a range from 16,000 to 400,000A/m, and more preferably in a range from 64,000 to 400,000 A/m.

When Fe--Cr--Co type magnetic alloys are used for formation of the thinstrap, the above-described age-hardening under magnetization cannotproduce sufficient difference in concentration between the highlymagnetic and nonmagnetic phases. In order to cover this deficit inspinodal decomposition, secondary age-hardening is usually applied tothe strap after taken up onto the tight roll. In connection with this,such secondary age-hardening should not precede taking-up onto the rollsince the advanced application of the secondary age-hardening wouldimpair good workability possessed by the strap just after the primaryage-hardening and would seriously hinder smooth formation of the tightroll.

After the final age-hardening, the magnetized strap 1' is taken uptightly onto a roll which may be either round as shown in FIG. 2 orsquare as shown in FIG. 3 in cross sectional profile. In either case, arolled strap 7 has a center space. In the structure of the rolled strap7, one face 1a' of the strap 1' is located near the periphery of therolled strap 7 and the other face 1b' of the strap 1' is located nearthe core of the rolled strap 7. As a consequence, the rolled strap 7 hasradial magnetic anisotropy which is indicated with dot-line arrows inthe illustration.

As remarked above, a secondary age-hardening is applied to such a rolledstrap when the strap is made of Fe--Cr--Co type magnetic alloys. Thissecondary age-hardening is employed for the purpose of enlarging thedifference in concentration between the highly magnetic and nonmagneticphases, thereby making the magnetic anisotropy intenser. Preferably, thesecondary age-hardening should be carried out in a temperature rangefrom 620° to 500° C. with gradual lowering in temperature. Succhtemperature lowering may be carried out either continuously or stepwise.Further, the secondary age-hardening may be carried out either with orwithout magnetization. When magnetization is adopted, the direction ofmagnetic flux in the field should meet the radial direction of therolled strap. In addition to increase in intensity of magneticanisotropy, application of the secondary age-hardening removes strainwhich was developed during formation of the tight, rolled strap.

After the final age-hardening, the rolled strap is further magnetized sothat different poles should be located on the different radial endsections of a permanent magnet 8 as shown in FIGS. 4 to 6. In theexamples shown in FIGS. 4 and 5, same poles are located on a same radialend section of the permanent magnet 8. Whereas, in the example shown inFIG. 6, different poles are alternately located on a same radial endsection of the permanent magnet 8. It is only required that, on a commonradial line, different poles should be located on the different radialend sections of the permanent magnet 8.

As a result of the secondary age-hardening, there are produced lots ofhighly magnetic phase fractions in the magnetic alloy and they arehighly elongated in the radial direction of the roller strap. By furthermagnetizing such a rolled strap having significant radial magneticanisotropy, the resultant permanent magnet has excellent magneticcharacteristics in its radial directions.

EXAMPLE

A Fe--Cr--Co type magnetic alloy was used for preparation of a thin,elongated strap having 0.2 mm. thickness and 100 mm. width. The alloycontained 24% by weight of Cr, 12% by weight of Co, and Fe substantiallyin balance. An ingot formed by vacuum casting was subjected to hotforgoing, hot rolling and cold working in order to be deformed into thestrap. The strap was them passed, at a rate of 7 m/min, through afurnace of 5 m. length which had a uniform temperature section (1050°C.) of 1 m. length and was filled with hydrogen gas, for continuousannealing.

Primary age-hardening of the strap was carried out on an apparatus shownin FIG. 1. In heating of the strap, the strap was first heated at 700°C. for 30 minutes, the temperature was lowered down to 610° C. at a rateof 40° C./hr, and the temperature was further rapidly lowered down tothe room temperature. The magnetic field was set to 150,000 A/mintensity.

Next, the strap was split to 2.0 mm. width and taken up onto a hollowroll of 8 mm. inner diameter, 25 mm. outer diameter and 2 mm. thickness.

The rolled strap was then subjected to secondary age-hardening in whichthe rolled strap was first heated at 650° C. for 60 minutes, and thetemperature was next lowered down to 500° C. at a rate of 5° C./hr.After the age-hardening, the rolled strap was further magnetized inorder to obtain a hollow, permanent magnet disc having different poleson different radial end sections.

The magnet disc so produced was incorporated in a loudspeaker magneticcircuit as shown in FIG. 7. More specifically, the magnetic circuitincluded a yoke 9, a center pole 10 of the yoke 9, a voice coil 11 woundabout the center pole 10 and a vibration plate 12 arranged over the yoke9. The above-described permanent magnet disc 8 was arranged atop theyoke 9 surrounding the voice coil 11 on the center pole 10. The yoke 9had an outer diameter (R) of 27 mm and a thickness (t) of 1.5 mm.

On such a magnetic circuit magnetic flux density Bg was measured at acircular gap 13 between the inner periphery of the permanent magnet disc8 and the voice coil 11, and the measured value was equal to 1.0 T.

A like permanent magnet disc was prepared using same Fe--Cr--Co typealloy but without any application of age-hardening, and the permanentmagnet disc 8 in FIG. 7 was replated by this sample for comparison. Themagnetic flux density taken at the same place on the magnetic circuitamounted to 0.54 T.

From comparison of the data, it is quite evident that the radiallyanisotropic magnet produced in accordance with the present inventionpossesses excellent magnetic characteristics.

It is also clear from FIG. 7 that the height H of the magnetic circuitcan be reduced by arrangement of the permanent magnet disc 8 atop theyoke 9. This assures very compact construction of the loudspeaker.

We claim:
 1. A method for producing a magnet having radial magneticanisotropy comprising the steps offorming a thin, elongated strap from aCu--Ni--Fe spinodal decomposition type alloy which can be provided withmagnetic anisotropy, subjecting said strap to age-hardening underconcurrent magnetization in its thickness direction, and taking up saidstrap, after age-hardening, into a light, hollow roll of a prescribedshape.
 2. A method for producing a magnet having radial magneticanisotropy comprising the steps offorming a thin, elongated strap from aFe--Cr--Co spinodal decomposition type alloy which can be provided withmagnetic anisotropy, subjecting said strap to age-hardening underconcurrent magnetization in its thickness direction, and taking up saidstrap, after age-hardening, into a tight, hollow roll of a prescribedshape, subjecting said roll of said strap to additional age-hardening.3. A method as claimed in claim 1 further comprising the stepofsubjecting said roll of said strap to additional age-hardening.
 4. Amethod as claimed in claim 2 in whichsaid Fe--Cr--Co type magnetic alloycontains 2 to 30% by weight of Cr, 5 to 37% by weight of Co, and Fesubstantially the balance.
 5. A method as claimed in claim 4 inwhichsaid Fe--Cr--Co type magnetic alloy further contains 0.1 to 8% byweight in full of at least one of Ti, Zr, Ni, V and Si.
 6. A method asclaimed in either of claims 3 or 2 in whichsaid first-namedage-hardening is carried out at a temperature conducive to spinodaldecomposition of said magnetic alloy.
 7. A method as claimed in claim 6in whichsaid first-named age-hardening is carried out by heating saidstrap at a temperature in a range of 670° to 720° C. for a period of 10to 60 minutes, lowering the temperature down to 600° to 620° C. at arate of 10° to 90° C./hr, and quickly cooling the strap.
 8. A method asclaimed in either of claims 3 or 2 in whichintensity of the magneticfield at said first-named age-hardening is in a range from 16,000 to400,000 A/m.
 9. A method as claimed in either of claims 3 or 2 inwhichsaid second-named age-hardening is carried out by graduallylowering the heating temperature of said roll of said strap from 620° to500° C.
 10. A method as claimed in either of claims 3 or 2 in whichsaidsecond-named age-hardening is performed with concurrent magnetization.11. A method as claimed in either of claims 3 or 2 in whichsaidsecond-named age-hardening is performed without magnetization.
 12. Amethod as claimed in claim 8 in which intensity of the magnetic field atsaid first-named age-hardening is in the range from 64,000 to 400,000A/m.
 13. A method as claimed in claim 7 in whichsaid second-namedage-hardening is performed without magnetization.
 14. A method forproducing a magnet comprising the steps offorming a thin, elongatedstrap from a Cu--Ni--Fe spinodal decomposition type magnetic alloy whichcan be provided with magnetic anisotropy, said alloy containing 10 to30% by weight of Ni, 10 to 30% by weight of Fe and the balance beingsubstantially Cu, subjecting said strap to age-hardening underconcurrent magnetization in its thickness direction, and taking-up ofsaid strap after said age-hardening onto a tight, hollow roll of aprescribed shape.
 15. A method as claimed in claim 14 further comprisingthe step of subjecting said roll of said strap to additionalage-hardening.
 16. A method as claimed in claim 15 in which said alloycontains 15-25% by weight of Ni, 15-25% by weight of Fe and the balancebeing substantially Cu.